With stricter environmental regulations on transportation fuels, coupled with higher level of heavier crude oil processing, demand for hydrogen in refineries is expected to increase in order to meet the need for more hydrotreating and hydrocracking processes. Today, about 95% of hydrogen is supplied by large-scale steam reforming of natural gas, which employs reforming, water-gas-shift (WGS), and pressure adsorption, processes in successive steps. Efficiency, however, suffers for smaller scale operations. Therefore, hydrogen production at a smaller scale from fossil fuels necessitates further development to meet the requirements of purity, economics and versatility for a variety of emerging applications such as the use of fuel cell powered vehicles.
During the last decade, there have been growing concerns about the shortage of energy supply in the world. Fuel cell technologies that use hydrogen may offer potentially non-polluting and efficient fuel for today's rising energy demands. While this type of technology is in development for renewable sources of hydrogen, hydrogen is typically commercially produced through conversion of hydrogen rich energy carriers.
Therefore, cost effective methods for the production of hydrogen from fossil fuels are becoming increasingly important, and more particularly, methods for the production of low cost, pure hydrogen, which is free from carbon monoxide are needed.
Hydrocarbon steam reforming (SR), partial oxidation (PDX) and autothermal reforming (ATR) are important known processes for hydrogen production. Steam reforming of high molecular weight hydrocarbons has been practiced for over 40 years in locations where natural gas is not available. ATR of higher hydrocarbons, however, is preferable to SR because it combines a highly endothermic SR process with an exothermic PDX process. This means that ATR is a more thermally stable process than SR and can be driven to thermo-neutral conditions. In addition, the start up of the ATR is more rapid than SR.
Noble metal based catalysts, particularly Rh-based catalysts, are active in the reforming of hydrocarbons to synthesis gas. Unlike conventional nickel catalysts, the Rh-based catalysts tolerate the presence of sulfur and prevent coke deposition, these being desirable properties for any reforming catalyst that is used with a commercial fuel as a feedstock. Commercial fuels typically contain sulfur that cannot be completely removed, as the sulfur may be present in an aromatic ring. Furthermore, with certain commercially available catalysts, sulfur deposition can also induce coke accumulation. Another significant drawback to catalysts that include noble metals is their cost, as compared with non-noble metal based catalysts. For example, the price of rhodium has risen almost 10 fold over the past 3 years from an average of about USD $16.77 per gram to about $149.71 per gram in 2010.
Steam reforming of methane is typically used as a form of catalytic reaction for the production of hydrogen. Conventional catalytic systems utilizing either nickel or noble metal catalysts for the steam reforming of methane require primary reaction temperatures of the order of about 700° C. and above, followed by rather extensive and expensive purification processes to provide a hydrogen product with reasonable purity (i.e., greater than 95% by volume) that can be used as a feed stock for many common processes.
Reforming of hydrocarbons other than methane, notably oil products, can be achieved with reactions similar to steam reforming of natural gas. The successive breaking of the C—C terminal bonds of higher hydrocarbons (i.e., hydrocarbons having at least 2 carbon atoms present) with suitable catalysts is more difficult than for methane; however, due to differences in reaction rates and increased propensity for thermal cracking (also called pyrolysis). To avoid the issues related to differing reaction rates and thermal cracking, carbon stripping is usually done in a separate pre-reformer, making this option of producing hydrogen more complex and more expensive than natural gas reforming.
Hydrogen can also be produced from non-volatile hydrocarbons such as heavy oils by gasification or partial oxidation. Gasification processes utilize steam at temperatures above about 600° C. to produce hydrogen whilst carbon is oxidized to CO2. Gasification, however, is usually not economical compared to steam reforming and partial oxidation processes. By comparison, partial oxidation is a considerably more rapid process than steam reforming. In partial oxidation, the hydrocarbon feed is partially combusted with oxygen in the presence of steam at a temperature of between about 1300° C. and 1500° C. Pressure has little effect on the reaction rate of the process and is usually performed at pressures of about 2 to 4 MPa to permit the use of more compact equipment and compression cost reduction. When air is used as the oxygen source, nitrogen must be removed from the resulting hydrogen gas product, typically requiring a separate stage following the oxidation reactor. Partial oxidation is thus more suitable for small scale conversion, such as in a motor vehicle that is equipped with fuel cells. The process can be stopped and started, as required for on-board reformation, and when in progress, it can provide elevated temperatures that may start steam reforming along with the oxidation processes. This is called autothermal reforming and involves all the reactions mentioned so far.
The water-gas-shift (WGS) reaction is an alternative hydrogen production technology frequently used following a primary catalytic reaction utilized to remove carbon monoxide impurities and increase overall hydrogen yield. The WGS reaction is mildly exothermic and is thus thermodynamically favored at lower temperatures. The kinetics of the reaction, however, are superior at higher temperatures. Therefore, it is common practice to first cool the reformate product from the reformer in a heat exchanger to a temperature between about 350° C. and 500° C., and then conduct the reaction over a suitable WGS catalyst. The resulting reformate is then cooled once again to a temperature between about 200° C. and 250° C. and then reacted on a low temperature designated WGS catalyst. Due to these several conversion and heat exchanging steps involved, however, the process is economically expensive and highly inefficient.
Pressure Swing Adsorption (PSA) is a well-known established method to separate hydrogen from a stream containing impurities. PSA employs multiple beds, usually two or more, of solid adsorbents to separate the impure hydrogen stream into a very pure (99.9%) hydrogen product stream and a tail gas stream that includes the impurities and a fraction of the hydrogen produced. As an example, synthesis gas (H2 and CO) may be introduced into one bed where the impurities, rather than the hydrogen, are adsorbed onto the adsorbents. Ideally, just before complete loading is achieved, this adsorbent bed is switched offline and a second adsorbent bed is placed on line. The pressure, on the loaded bed is subsequently reduced, which liberates the adsorbed impurities (in this case predominantly CO2) at low pressure. A percentage of the inlet hydrogen, typically approximately 15 percent, is lost in the tail gas. A significant disadvantage of the PSA is that low tail gas pressure essentially limits the system to a single WGS stage. Limiting a hydrogen separation system to a single WGS stage thus decreases the amount of CO conversion as well as the total amount of hydrogen recovery. PSA is also undesirable, as compared to the use of membranes, in part due to the mechanical complexity of the PSA, which leads to higher capital and operating expenditures and potentially increased downtime.
Hydrogen is produced and removed through hydrogen permeable metal membranes, such as palladium or palladium, alloys. Metallic membranes, particularly palladium or palladium alloys, however, are very expensive, sensitive to sulfur compounds, and difficult to co-sinter with or sinter onto a catalyst layer. Additionally, such devices produce hydrogen using only the WGS reaction.
Another type of membrane is the so-called dense protonic ceramic membrane for hydrogen separation and purification. It is based on the use of single-phase and mixed-phase perovskite-type oxidic protonic ceramic membranes for separating or decomposing hydrogen containing gases or other compounds to yield a higher value product. However, these membranes suffer many of the shortcomings noted previously.
Thus, it is highly desirable to develop membrane reactors that are capable of converting liquid hydrocarbons into high purity hydrogen in a single step.